This invention relates to display control technology. More particularly, it relates to a technique which is especially effective when applied to a control system for a display device, such as a CRT display, for example, a display control system in a CRT controller constituting a graphic display system as in a personal computer.
Heretofore, various CRT controllers for controlling CRT displays instead of MPUs have been proposed as LSIs each of which serves as a graphic display system as in a personal computer furnished with the CRT display and having a graphic video progressing function.
An example of a CRT Controller is manufactured by Hitachi Ltd. and includes therein a graphic pattern RAM (random access memory) of, e. g., 16.times.16 bits with which a function called `tilting` can be performed in which a pattern drawn in the RAM is repeatedly displayed on the desired area of the display screen of a CRT display to smear out the area. (Refer to "HD63484ACRTC Users' Manual" published in June, 1984 by Hitachi Ltd.)
The CRT controller having a RAM as described above (hereinbelow, called a `pattern RAM`) has the advantage that the tiling with any desired pattern can be performed at high speed. However, the capacity of the pattern RAM which can be built in the CRT controller cannot be considerably enlarged because of the limitation of the chip size. It is therefore impossible to keep a very complicated pattern written in the pattern RAM. Moreover, since color information is handled with the two values of "1" and "0", the number of colors which can be used at a time is restricted to two, and the pattern becomes monotonous. A further drawback is that, each time the pattern is changed, an MPU or the like needs to externally draw a desired pattern in the internal pattern RAM, so that the burden on the MPU is heavy.